CO2 is a gas produced as a by-product, in large quantities, in certain industrial operations, e.g. the manufacture of ammonia, in power plants that burn fossil fuels such as coal, oil or gas, and in a hydrogen plant that produces hydrogen by reforming of a hydrocarbon feedstock. Release of this by-product into the atmosphere is undesirable as it is a greenhouse gas. Much effort has been made towards the development of methods for the disposal of CO2 in a way other than release to the atmosphere. One technique of particular interest is subsurface disposal of CO2 into caverns or porous strata. Beneficial advantage of disposal can be realised if the CO2 is injected into a porous and permeable hydrocarbon reservoir where the injected CO2 serves to drive hydrocarbons (e.g. oil) in the reservoir towards a production well (a well from which hydrocarbons are extracted) thereby achieving enhanced oil recovery.
The injection of CO2 specifically for the purpose of increasing hydrocarbon production, known as Enhanced Oil Recovery (EOR) or “assisted recovery”, is described, for example, in US 2002/0036086. As described in this document, CO2 recovered from a production well can be reinjected via an injection well to reduce the viscosity of the petroleum in place in a formation, increasing its mobility and facilitating its recovery. However, the produced CO2 comprises a number of hydrocarbon impurities which affect the saturation pressure of the formation. The process of US 2002/0038086 is directed to a process for removal of such components from the produced CO2 prior to its reinjection. Further, US 2002/0036086 teaches that sequestration of CO2, which inherently occurs in such a process, is actually a drawback of the process, resulting in a “loss” of CO2 and requiring addition of further CO2.
Although CO2 injection in hydrocarbon reservoirs is therefore known for both enhanced oil recovery and for CO2 storage, in general the requirements for CO2 storage compared to CO2 use for enhanced oil recovery are significantly different.
Thus, for enhanced oil recovery, as taught by US 2002/0036086, any CO2 storage that occurs is considered a “loss” and is a disadvantage. In contrast, in CO2 sequestration it is desirable for as much of the injected CO2 as possible to remain in the reservoir.
Thus, in general, CO2 which it is desired to sequester should be in a form which is not miscible with the hydrocarbons in the reservoir, whilst the opposite is true for CO2 injected for the purpose of enhanced oil recovery. Similarly, it is desired that CO2 for sequestration is in a state which maximises storage, in particular being in the form of a high density material that has the maximum number of moles of CO2 per unit volume. In contrast in enhanced oil recovery it is generally desired to achieve recovery using the minimum volume of CO2 effective for the purpose.
Further, the quantities of CO2 involved in disposal by injection into a hydrocarbon reservoir are immense, generally of the order of millions of tonnes, and significantly more than the amounts involved in enhanced oil recovery. Not least because of the very large volumes involved the injection facility must be capable of reliably injecting the imported CO2. In other words, there must be continuous injection availability for all of the imported CO2. Furthermore, the reservoir must securely store the injected CO2, generally, for a period of time of at least 1000 years.
In general, it would be desired, if possible, that injected CO2 for sequestration is introduced into the reservoir at a significant distance from any production well and in a form which is not miscible with the hydrocarbons therein to maximise the storage capacity and minimise the transport of the injected CO2 to the production well. However, the distance of any injection wells from any production wells may be limited, for example by infrastructure limitations. In any case, with time the large volume of injected CO2 will start to result in an increase in the amount of produced vapour comprising carbon dioxide recovered from the production well. This CO2 can have a beneficial effect on oil recovery, but in the process of the present invention, enhanced hydrocarbon recovery is of lower priority than reliably maintaining CO2 injection availability and securely storing the injected CO2.
Further, the increased amount of produced CO2 with time must be dealt with. Although production wells will often have associated gas injection wells for the purpose of injection of produced vapour comprising CO2, even in the absence of CO2 sequestration, the increasing quantities of vapour comprising CO2 resulting from CO2 sequestration in the reservoir may saturate such facilities, leading to the requirement for more injection wells to be drilled and the cost associated therewith, all for a reservoir that may be well past its peak production.